Process for the manufacture of enriched colloidal silver

ABSTRACT

Colloidal silver is prepared in enriched form, suitable for the manufacture of photographic antihalation and color filter layers, from an aqueous medium in which the silver is present as a dilute dispersion, by adding a flocculating agent and separating the flocculate at a pH from 3 to 8 and redispersing it in the presence of citrate ions.

0 United States Patent 1113,615,789

[72] Inventor Heinrich Schaller [51] Int. Cl C08h 7/00 Fribourg, Switzerland 501 Field Search 106/135,

21 AppLNo. 864,932 137;75/1os 22 Filed Sept. 15, I969 [45] Patented Oct. 26, 1971 References Cited 73 Assignee Ciba Limited UNITED STATES PATENTS Bmbswimflmd 3,366,482 l/1968 Schalleretal. 96/94 Prwmy 1182811966 3,334,995 8/1967 Gaspar 75/108 [33] Switzerland [3 1] 9348/66 Primary Examiner-Morris L1ebman Continuation-impart of application Ser. No. 649,217, June 27, 1967, now abandoned.

[54] PROCESS FOR THE MANUFACTURE OF ENRICHED COLLOIDAL SILVER 5 Claims, 3 Drawing Figs.

52 us. c1 106/135,

Assistant Examiner-T. Morris Attorneys-Harry Goldsmith, Joseph G. Kolodny and Mario A. Monaco ABSTRACT: Colloidal silver is prepared in enriched form, suitable for the manufacture of photographic antihalation and color filter layers, from an aqueous medium in which the silver is present as a dilute dispersion, by adding a flocculating agent and separating the flocculate at a pH from 3 to 8 and redispersing it in the presence of citrate ions.

Cross-Reference This application is a continuation-in-part of Applicants copending application Ser. No. 649,217, filed June 27, 1967, the disclosure of which is relied on and incorporated by reference in this application.

It is known that silver halide-gelatine dispersions for photographic purposes can be advantageously freed from the extraneous salts formed in their manufacture and can be enriched by the so-called flocking-out process. Likewise, numerous flocculating agents suitable for this purpose are known.

As in the case of the silver halides, it is also necessary in the case of colloidal silver required for the manufacture of photographic antihalation and color filter layers, to enrich the dispersed silver and to free it from extraneous substances, especially from excess reducing agent and from byproducts formed by the reduction of silver salt. To achieve this object it is customary to reduce the silver salt in dilute aqueous gelatine to colloidal silver, allow the gelatine to solidify and then comminute and treat it with water. This method, which as is known can also be applied to the manufacture of silver halides, is not only time consuming but in the case of colloidal silver it leads to dispersions having a relatively low silver content since the concentration of the reducing solution can not be chosen at will.

When attempting to flocculate colloidal silver by the methods known'for silver halides, the following difficulties arise: In some cases the colloidal silver is very finely particulate so that only a part of it is taken up by the precipitating gelatine and unacceptably high losses result. Even when this disadvantage does not arise and flocculation can be carried out more or less completely, the flocculate can no longer be redispersed or the colloidal silver changes color during redispersion, for example from black to brown, which makes it unsuitable for the use in hand.

The present invention now consists in a process for the manufacture of enriched colloidal silver from an aqueous medium in which the colloidal silver has been obtained as a dilute dispersion by reduction of silver salts, when said medium is treated with an agent capable of flocculating colloidal silver at a pH value from 3 to 8, and the flocculate is then separated at a pH value from 3 to 8 and redispersed in enriched form in a protective colloid in the presence of an alkali group citrate.

The aqueous media used as starting materials, which contain finely dispersed metallic silver and a protective colloid, for example, egg albumen or preferably gelatine, can be prepared in the usual manner; depending on the reducing agent and working conditions used, silver particles of different sizes are obtained and therefore dispersions of different colors result. Thus, for example, blue-black colloidal silver is obtained when silver nitrate is reduced with hydrazine in a medium containing an alkali metal hydroxide. Yellow colloidal silver can be prepared from silver nitrate with the aid of tannin. Further suitable reducing agents are dextrine and hydroquinone.

Flocculating agents which come into consideration are those products capable of flocking-out silver halide emulsions which cause flocculation at a pH value from 3 to 8. Depending on the nature of the colloidal silver dispersions to be flocked out certain flocculating agents may produce particularly favorable results. Thus for example for flocking out the abovementioned blue-black colloidal silver reduced with hydrazine those colorless compounds are especially suitable that contain at least one acid group imparting solubility in water, at least one six-membered heterocycle consisting of three or four carbon atoms and three or two nitrogen atoms, and at least one aromatic residue. Such compounds are known (see British Specification No. 1,041,085).

As further flocculating agents which, for example, may be advantageously used for flocking out yellow colloidal silver reduced with tannin, there may be mentioned those compounds which are obtained by condensing a diaminodiphenylsulfonic acid with a thiophene-Z,S-dicarboxylic acid dihalide in the molecular ratio of 1:2 to 2:1, particularly the condensation products of diaminodiphenylsulfonic acids of the formula X HOzS in which X stands for a methyl group or for a hydrogen atom. The condensation of the diamino compounds with the acid halides may be carried out in an aqueous medium, advantageously in the presence of an acid acceptor in such a manner that the pH value of the reaction medium does not fall below 7. It is also advantageous to add the acid halidepreferably thiopene-2,5-dicarboxylic acid dichloride-to the reaction mixture as a solution in a water-miscible organic solvent, for example, in acetone.

In the present process the flocculate is separated at a pH value from 3 to 8. It is advantageous to adjust the pH value after the reduction and before the addition of the flocculating agent to the desired value within these limits, whereupon the flocculating agent is added. Alternatively, the flocculating agent may first be added at a pH value outside this range and the pH value adjusted afterwards. Even when the flocculating agent is added at a pH value from 3 to 8 it may sometimes be advantageous to vary the pH value within the indicated limits before proceeding to the separation of the flocculate. The pH value may be varied by conventional means, for example strong acids, for example sulfuric acid, strong bases, for example sodium hydroxide or, if desired, compounds having a buffer action, for example weak acids, weak bases, acidic or basic salts.

The amount of flocculating agent used should suffice to flock out as much of the colloidal silver as possible. The amount depends'on various factors, for example the concentration of the silver dispersion, the content of colloidal (gelatine), reducing agent, byproducts and further additives. and it may differ considerably from case to case. If necessary. it is easy to determine the minimum and/or maximum quantity by preliminary tests. A large excess is in general neither deleterious, nor advantageous.

The fiocculate may be separated in the usual manner, for example by filtration, or most simply by allowing it to settle and then decanting the supernatant liquid. In the case of flocculate that settle slowly, separation can be accelerated by centrifuging. To ensure that as much as possible of the water-soluble extraneous substances also contained in the flocculate are removed, it is recommended to wash the flocculate with water.

According to this invention the flocculate obtained in this manner is redispersed in a protective colloid in the presence of citrate ions. In this use, too, the most advantageous protective colloid is gelatine. In some cases the protective colloid contained in the fioceulate sufiices for the redispersion, but in most cases an addition of further protective colloid is advantageous. For the redispersion it is advantageous to choose a medium pH value, for example from 5 to 7. Therefore, the citrate ions are introduced into the mixture by addition of a water-soluble alkali group citrate such as ammonium, lithium, rubidium, caesium and preferably sodium or potassium citrate. Otherwise the desired pH value can be adjusted in the usual manner, for example when the medium is too acidic by adding for example an alkali metal hydroxide. In general it is of advantage to carry out the redispersion at a temperature from 40 to 60 C.

The flocculate is redispersed in such a manner that the col' loidal silver is enriched compared with its content in the reduction mixture. The amounts of substances required for the dispersion may be so chosen, for example, that a material immediately suitable for use in the casting of photographic filter or antihalation layers is directly obtained. It is also possible to manufacture dispersions having a higher silver content which, before being used are diluted to the requisite concentration by adding water and/or gelatine thereto. In general, it may be said that the dispersions advantageously contain 1 to 10 percent of metallic silver and 4 to 10 percent of a protective colloid, preferably gelatine. The proportion of citrate ions is variable and may be, for example, 3 to 8 percent (calculated as HOOC-CH,-C(OH)(COOI-l)-CH,-COOH) referred to dry gelatine).

The silver dispersions thus obtained may be worked up in the usual manner and with the usual additives, for example wetting agents, potassium bromide for adjusting an optimal pBr value, and hardeners for the gelatine, to form photographic layers.

The following examples illustrate the invention.

EXAMPLE 1 A solution of 60 g. of gelatine and 120 g. of citric acid in 3 liters of water is heated to 40 C. and, with mechanical stirring, mixed with a solution of 24 ml. of 100 percent hydrazine hydrate in 90 ml. of aqueous 28 percent sodium hydroxide solution. A solution of 300 g. of silver nitrate in 3 liters of water is then added within 10 minutes while maintaining the temperature at 40 C. Then 60 ml. of 28 percent sodium hydroxide solution and after 25 minutes another 70 ml. of 28 percent sodium hydroxide solution are added and the batch is allowed to react for another 25 minutes; 160 ml. of a percent aqueous solution of the sodium salt of the sulphonic acid of the formula are then added. The gelatine contracts in fioccular form together with the silver. The supernatant, clear solution is decanted and the flocculate is washed with 3X6 liters of water. The resulting flocculate is redispersed by stirring for 30 minutes at 50 C. in a solution containing 380 g. of dry gelatine, 320 ml. of a percent aqueous sodium citrate solution (pH value; 6.0) in 4 liters of water. The silver dispersion obtained in this manner is of a strong blue-black color.

FIG. 1 represents the absorption curve of the colloidal silver thus prepared in a layer lp. thick, cast on a transparent triacetate foil. The extinction is plotted on the ordinate and the wavelength in nanometers on the abscissa.

EXAMPLE 2 A solution of g. of lyalbic acid and 160 ml. of 10 percent aqueous sodium hydroxide solution in 29 liters of water is prepared at room temperature. A solution of 43 g. of silver nitrate in 180 ml. of water and a solution of 5.2 g. of tannin in 150 ml. of water are added, the whole is heated to 65 C. and allowed to react for 15 minutes. A solution of 40 g. of gelatine in 600 ml. of water is then added, and the pH value is lowered to 6.0 by adding 2N sulphuric acid. On addition of 750 ml. of a 5 percentsolution of the condensation product prepared as 75 described below the gelatine together with the finely disperse silver fiocculates. The flocculate is washed with 4X6 liters of water. The flocculate is then added to a solution of 200 g. of gelatine and 10 g. of sodium citrate in 2.1 liters of water. The pH value is adjusted to 7.0 by adding 2N sodium hydroxide solution and the flocculate is redispersed by stirring for 30 minutes at 50 CJThe resulting gelatine-silver dispersion may be used as a yellow filter layer in photographic materials.

FIG. 2 represents the absorption curve of the colloidal silver thus prepared in a layer 2y. thick on a transparent triacetate foil. The extinction is plotted on the ordinate and the wavelength in nanometers on the abscissa.

The condensation product used as flocculating agent is prepared in the following manner:

A mixture of 34.8 parts of 4,4-diaminodiphenyl-( l l ')-2,2 -disulfonic acid and 20 parts by volume of 30 percent sodium hydroxide solution in 300 parts of water at 25 C. is stirred so that a solution of the disodium salt is obtained. This solution is mixed with 40 parts of crystalline sodium tetraborate and, when it has dissolved, a solution of 11 parts of thiophene-2,5- dicarboxylic acid dichloride in 25 parts of acetone is added in portions, and the whole is stirred for 16 to 20 hours. On completion of the condensation the pH value should be at least 7; if it is lower, small amount of sodium carbonate is added. The reaction mixture is then heated to C, 225 parts of saturated potassium acetate solution are added, and the batch is stirred for 10 to 12 hours. A slightly gelatinous precipitate forms which is filtered, stirred with 400 parts of ethanol, the suspension thus obtained is stirred for 1 hour. again filtered and dried at C. About 60 parts of a whitish powder are obtained which gives a practically neutral solution in water.

EXAMPLE 3 A solution of 60 g. of gelatine and I20 g. of citric acid in 3 liters of water is heated to 40 C. and a solution of 24 ml. of hydrazine hydrate percent) in 90 ml. of 28 percent aqueous sodium hydroxide solution is stirred in. Then, within 10 minutes, a solution of 300 g. of silver nitrate in 3 liters of water is added, while maintaining the temperature at 40 C. Subsequently ml. of 28 percent sodium hydroxide solution are added and the batch is allowed to react for 40 minutes at 40 C., and then ml. of a 5 percent aqueous solution of the sodium salt of the sulfonic acid of the formula (2') are added. The gelatine is fiocculated together with the dispersed silver. The supernatant clear liquid is decanted and the fiocculate washed with 3X6 liters of water. The flocculate obtained in this manner is redispersed by stirring for 30 minutes at 50 C. in a solution of 380 g. of gelatine, 320 ml. of 10 percent aqueous sodium citrate solution (adjusted to pH=6.0) in 4 liters of water. The resulting silver dispersion is a brownish-black color. v

FIG. 3 represents the absorption curve of a layer of 2p thickness cast on a transparent triacetate foil. The extinction is plotted on the ordinate and the wavelength in nanometers on the abscissa.

I claim:

1. A process for the manufacture of enriched colloidal silver from an aqueous medium which contains a protective colloid selected from the group consisting of gelatine and egg albumen in which the colloidal silver has been obtained as a dilute dispersion by reduction of a silver salt selected from the group consisting of silver nitrate and silver halide. which process comprises adding to said medium an agent that flocculates the colloidal silver at a pH value from 3 to 8, separating the flocculate at a pH value from 3 to 8 and redispersing it in enriched form in said protective colloid in the presence of an alkali group citrate.

2. A process according to claim 1 for the manufacture of enriched colloidal silver from an aqueous medium in which the colloidal silver has been obtained as a dilute dispersion by reduction of a silver salt selected from the group consisting of silver nitrate and silver halide, which process comprises adding to said medium an agent that flocculates the colloidal silver at a pH value from 3 to 8, separating the flocculate at a pH value from 3 to 8 and redispersing it in enriched form in aqueous gelatine in the presence of an alkali group citrate.

3. A process according to claim 1 for the manufacture of enriched colloidal silver from an aqueous medium in which the colloidal silver has been obtained as a dilute dispersion by reduction of a silver salt selected from the group consisting of silver nitrate and silver halide, which process comprises adding to said medium as a flocculating agent a colorless compound containing at least one acid group imparting solubility in water, at least one six-membered heterocycle consisting of three to four carbon atoms and two to three nitrogen atoms, and at least one aromatic residue, separating the flocculate at a pH value from 3 to 8 and redispersing it in enriched form in aqueous gelatine in the presence of an alkali group citrate.

4. A process according to claim 1 for the manufacture of enriched colloidal silver from an aqueous medium in which the colloidal silver has been obtained as a dilute dispersion by reduction of a silver salt selected from the group consisting of silver nitrate and silver halide, which process comprises adding to said medium as a flocculating agent a colorless compound prepared by condensing a diamine of the formula where R, represents an aromatic residue containing sulfonic acid groups, with a dicarboxylic acid, separating the flocculate at a pH value from 3 to 8 and redispersing it in enriched form in aqueous gelatine in the presence of an alkali group citrate.

5. A process according to claim 1 for the manufacture of enriched colloidal silver from an aqueous medium in which the colloidal silver has been obtained as a dilute dispersion by reduction of a silver salt selected from the group consisting of silver nitrate and silver halide, which process comprises adding to said medium as a flocculating agent a colorless com pound obtainable by condensing a diamine of the formula 

2. A process according to claim 1 for the manufacture of enriched colloidal silver from an aqueous medium in which the colloidal silver has been obtained as a dilute dispersion by reduction of a silver salt selected from the group consisting of silver nitrate and silver halide, which process comprises adding to said medium an agent that flocculates the colloidal silver at a pH value from 3 to 8, separating the flocculate at a pH value from 3 to 8 and redispersing it in enriched form in aqueous gelatine in the presence of an alkali group citrate.
 3. A process according to claim 1 for the manufacture of enriched colloidal silver from an aqueous medium in which the colloidal silver has been obtained as a dilute dispersion by reduction of a silver salt selected from the group consisting of silver nitrate and silver halide, which process comprises adding to said medium as a flocculating agent a colorless compound containing at least one acid group imparting solubility in water, at least one six-membered heterocycle consisting of three to four carbon atoms and two to three nitrogen atoms, and at least one aromatic residue, separating the flocculate at a pH value from 3 to 8 and redispersing it in enriched form in aqueous gelatine in the presence of an alkali group citrate.
 4. A process according to claim 1 for the manufacture of enriched colloidal silver from an aqueous medium in which the colloidal silver has been obtained as a dilute dispersion by reduction of a silver salt selected from the group consisting of silver nitrate and silver halide, which process comprises adding to said medium as a flocculating agent a colorless compound prepared by condensing a diamine of the formula H2N-R1-NH2 where R1 represents an aromatic residue containing sulfonic acid groups, with a dicarboxylic acid, separating the flocculate at a pH value from 3 to 8 and redispersing it in enriched form in aqueous gelatine in the presence of an alkali group citrate.
 5. A process according to claim 1 for the manufacture of enriched colloidal silver from an aqueous medium in which the colloidal silver has been obtained as a dilute dispersion by reduction of a silver salt selected from the group consisting of silver nitrate and silver halide, which process comprises adding to said medium as a flocculating agent a colorless compound obtainable by condensing a diamine of the formula 